Step away from the chips and put down the margarine. It turns out that trans fats may make you more irritable—a theory you may have experienced first hand if you’ve ever had a case of the post-donut grumps. According to a study conducted by the University of California, San Diego’s School of Medicine, the consumption of dietary trans fatty acids increases feelings of aggression, irritability and impatience.

If you’re not already aware, trans fats are usually found in processed foods after fats have been hydrogenated. Hydrogenated fats have hydrogen added to them to make them solid at room temperature and last longer without spoiling, so some of the worst offenders include stick margarine, shortening, store-bought chips and crackers, and fast food.

And trans fats are double trouble for your health. Trans fats not only raise your bad (LDL) cholesterol, they also lower your good (HDL) cholesterol. Both of these are bad news for your heart, and according to this new study, it could be bad news for your mood and relationships, too.

The study’s lead author, Dr. Beatrice Golomb, says, “If the association between trans fats and aggressive behavior proves to be causal, this adds further rationale to recommendations to avoid eating trans fats, or including them in foods provided at institutions like schools and prisons, since the detrimental effects of trans fats may extend beyond the person who consumes them to affect others.”

The best way to avoid trans fats in your diet is to become a label-reading fool (in the best sense of the word). Look for “partially hydrogenated vegetable oil” in commercially prepared baked goods, snacks and fried foods. And, unless the company specifically says otherwise, you can pretty much assume that any fried foods at a restaurant or fast food joint contain trans fats.

Small amounts of trans fats occur naturally in some dairy and meat products, but according to the Mayo Clinic, the ones in processed foods seem to have a more harmful effect than the naturally occurring ones.